Part IV · Molecular biology and therapeutic vulnerabilities · Chapter 15

HER family signaling

Amplification and activating mutation are two diseases sharing one gene name.

1 · Receptor dimerization, ligand dependence, and downstream output

The four members of the family, EGFR, HER2, HER3 and HER4, signal only as dimers. The identity of the dimerisation partner determines what the signalling produces. Nothing in this chapter makes sense if HER2 is treated as a receptor acting alone.

HER2 has no known ligand. Its extracellular region adopts a conformation resembling the ligand-activated state of the others, which leaves it permanently available for dimerisation. The crystal structure of the extracellular domain bound to pertuzumab showed that the antibody binds near the centre of domain II and sterically blocks the pocket required for dimerisation¹. That is a different mechanism from blocking a ligand. It is the reason pertuzumab adds to trastuzumab rather than duplicating it.

HER3 is the reciprocal case. Its kinase domain is essentially inactive, yet it carries the strongest coupling to PI3K of any family member. In HER2-amplified breast cancer cells, ablating HER3 blocks proliferation as effectively as ablating HER2, and constitutively active Akt rescues both. The pair functions as a single oncogenic unit².

Downstream output is not confined to the two canonical arms. HER2 signalling stabilises the loss of the cyclin-dependent kinase inhibitor p27³, which places it directly upstream of the cell cycle machinery in Cell cycle regulation. A nuclear pool of the receptor binds and transactivates the COX-2 promoter⁴, so some of its transcriptional effect does not pass through a cytoplasmic cascade at all.

The working conclusion is that HER2-directed therapy is dimer-directed therapy. Which partner is engaged determines both what the tumour depends on and which class of drug can remove it.

2 · Amplification, overexpression, and the expression continuum

Amplification is a statement about DNA. Overexpression is a statement about protein. They correlate, the correlation is incomplete, and the assays fail in different ways. That separation is developed in Genetic and protein-level heterogeneity and their imperfect correlation and is assumed here.

The clinical foundation is the observation that HER2 amplification correlated with relapse and with survival in primary breast cancer, which established the gene as prognostic before any drug existed against it⁵.

What has changed since is that the assay is being asked a question it was not built for. HER2 protein is present on normal breast epithelium. The immunohistochemical assay was validated to separate overexpression from its absence. It is now used to grade antigen abundance across the whole range, including the HER2-low category. The reproducibility cost of that repurposing is quantified in HER2-low and ultralow reframed as a heterogeneity problem, and the activity of conjugates at the bottom of the range is set out in Implications for conjugate payload delivery and the bystander effect and in DAISY⁶.

One point specific to this chapter deserves emphasis, because it is a biology finding disguised as a measurement problem. HER2 expression is hormonally modulated. Oestrogen and antioestrogen change HER2 messenger RNA and p185 protein in receptor-positive cell lines in opposite directions⁷, and short-term neoadjuvant endocrine therapy upregulates HER2 protein in tumours reported as hormone receptor-positive and HER2-negative⁸.

A HER2 result is therefore a measurement made at one moment under one hormonal exposure. It is not a fixed attribute of the tumour. A change in it after endocrine therapy is not automatically evidence of clonal change.

3 · ERBB2 amplicon architecture, extrachromosomal DNA, and instability

The mechanistic argument for why an amplified gene generates heterogeneity is made in Amplicon topology, extrachromosomal DNA, and the mechanistic origin of instability and is not repeated. The short form is that circular extrachromosomal elements carry no centromere, so copy number is redistributed unequally at every division rather than conserved.

Three additions belong here rather than there.

The first is generality, because extrachromosomal amplification is not a curiosity confined to one gene. A survey across 17 cancer types found extrachromosomal DNA in nearly half of tumours and almost never in normal cells. Driver oncogenes were the genes most commonly carried on it. Modelling in the same study predicted that this mode of amplification raises copy number and intratumoural heterogeneity more effectively than chromosomal amplification does⁹. Whole-genome sequencing from 3,212 patients gave the same picture with outcome attached. Oncogenes were enriched on extrachromosomal DNA, and transcription per copy was higher than for copy number-matched linear DNA. Patients whose cancers carried it had shorter survival after controlling for tissue type¹⁰.

The second is content, because an amplicon amplifies a neighbourhood rather than a gene. Whole-genome and transcriptome analysis of HER2-positive breast cancers shows the structural context in which ERBB2 amplification arises¹¹. Genes co-amplified by proximity are not under selection and may still contribute to phenotype, which is one reason HER2-positive disease is not uniform.

The third is what the topology predicts clinically. A tumour's HER2 copy number is a distribution across its cells rather than a single value. The width of that distribution is a property of how the gene came to be amplified. A single in situ hybridisation ratio reports the centre of the distribution and discards its width. Amplicons generated by breakage-fusion-bridge and those carried extrachromosomally behave differently under treatment, which is developed in Breakage-fusion-bridge and extrachromosomal amplification behave differently, and preclinical HER2-heterogeneous models show the corresponding subclonal dynamics during the evolution of resistance¹².

4 · HER2 activating mutations as a disease separate from amplification

Amplification and activating mutation produce different diseases. They are detected by different assays, they arise in different contexts, and they respond to different drugs. Treating them as one entity because they share a gene name is the single most consequential error in this area.

Somatic HER2 mutations occur in breast cancers that lack HER2 amplification. From eight breast cancer genome-sequencing projects, 25 patients with HER2 somatic mutations in non-amplified cancers were identified, and 13 of those mutations were characterised functionally. Seven were activating. One in-frame deletion had a neomorphic phenotype with increased phosphorylation of EGFR or HER3, and L755S conferred lapatinib resistance without being activating in those systems. All were sensitive to the irreversible inhibitor neratinib¹³.

Three consequences follow, and each one breaks an assumption imported from amplified disease.

The diagnostic route is different. These tumours are HER2-negative by immunohistochemistry and in situ hybridisation, because those assays measure protein abundance and copy number rather than kinase activity. Sequencing is the only way to find them Prognostic and predictive assays and somatic profiling.

The therapeutic logic is different. An antibody directed at the extracellular domain of a receptor present at normal density has very little antigen to act on. Antibody-based reasoning does not transfer. Kinase inhibition does, and allele identity matters within that. In the SUMMIT basket study of neratinib, activity in HER2-mutant cancers varied by both tumour type and mutant allele. The variation was larger than the preclinical models had predicted. The greatest activity was seen in breast, cervical and biliary cancers, and in tumours carrying kinase domain missense mutations¹⁴. Co-occurring gain-of-function mutations in HER2 and HER3 further modulate activation and inhibitor sensitivity¹⁵.

The context is different. Activating ERBB2 mutations were more than twice as common in endocrine-resistant tumours among 1,918 sequenced breast cancers, of which 692 had prior hormonal therapy exposure¹⁶. In hormone receptor-positive disease, a HER2 mutation is therefore often an acquired resistance mechanism rather than a founding driver. That makes it a chapter in Estrogen receptor signaling and endocrine resistance as much as one here, and the treatment problem it creates is taken up in HER2-mutant disease as a separate therapeutic problem.

5 · HER3, neuregulin, and ligand-driven escape

HER3 is where HER-family inhibition fails. It fails for a reason that is structural rather than accidental.

Because HER3 carries the PI3K coupling and HER2 supplies the kinase activity that phosphorylates it, the pathway's output depends on a transphosphorylation reaction rather than on autophosphorylation. Under HER-family tyrosine kinase inhibition, HER3 and downstream PI3K and Akt signalling recover. The recovery is driven by a compensatory shift in the HER3 phosphorylation and dephosphorylation equilibrium. Increased membrane HER3 drives the forward reaction, and reduced phosphatase activity impedes the reverse one. The whole shift is produced by Akt-mediated negative feedback¹⁷.

That paper also makes a measurement point that has largely been ignored. The biological marker of whether a HER-family inhibitor is working should be HER3 transphosphorylation rather than HER2 autophosphorylation. A drug can look fully effective on the target it was designed against while the output it was meant to shut down is restored.

Ligand supplies the other route. Neuregulin activates HER3 directly, from a source that need not be the tumour cell itself. Adipocyte precursor-derived NRG1 has been shown to drive resistance to receptor tyrosine kinase inhibition in urothelial carcinoma¹⁸. That finding is not in breast cancer. It is cited here as a paradigm for stromal ligand supply rather than as evidence about this disease.

Two further links run out of this section. Oestrogenic promotion of HER2 kinase activity in mammary tumour cells requires HER3 signalling¹⁹, which is the receptor arm feeding the dimer. And heregulin upregulates GPR30 through the HER2 and HER3 route²⁰, which is the dimer feeding the membrane oestrogen arm. Both of those observations point at ER and HER2 cross-talk, the bidirectional plane.

6 · p95HER2, splice variants, and epitope loss

Trastuzumab binds an epitope on the extracellular domain of the receptor. Anything that removes that domain while retaining kinase activity produces a tumour that is HER2-driven and antibody-resistant from the outset.

p95HER2 is an amino-terminally truncated receptor with kinase activity that cannot bind trastuzumab. Cells engineered to express it remained sensitive to lapatinib, which inhibited p95HER2 phosphorylation and downstream Akt and MAPK signalling, and the comparison was extended to patients with advanced disease using a paraffin-based immunofluorescence assay²¹. The truncated receptor also has consequences for the other pathway in this chapter, because p95 and the related 611-CTF fragment downmodulate the oestrogen receptor in HER2-positive breast cancers²².

Splice variation of the same gene produces a second class of epitope problem. HER2 delta 16 lacks exon 16 and dimerises constitutively, and its effect is larger than a rise in signalling output. It directs luminal cell identity and oestrogen receptor signalling in HER2-positive breast cancer²³. A splice variant of a receptor tyrosine kinase is therefore acting as a lineage determinant, which is not what the classical model of this pathway would predict.

The measurement point is simple and is rarely stated in a report. An antibody raised against the intracellular domain of HER2 will score a truncated receptor as present. An antibody raised against the extracellular domain will not. A positive immunohistochemistry result is a statement about the epitope the assay recognises, and the epitope the assay recognises is not always the epitope the drug needs.

Epitope loss also happens under treatment rather than only at baseline, and when it does it is indistinguishable in a single report from a tumour that was always heterogeneous. That argument is made in HER2 loss as an acquired resistance mechanism.

7 · ER and HER2 cross-talk, the bidirectional plane

Cross-talk is usually drawn as a single arrow running from HER2 into the oestrogen receptor. That picture is half of the system. The half it omits changes what should be expected under treatment.

HER2 into the receptor. HER2 kinase signalling targets the receptor and promotes hormone-independent growth²⁴. Tamoxifen-resistant tumours show increased cross-talk between the two²⁵. HER2 activates the coactivator AIB1 by phosphorylation, and tumours high for both AIB1 and HER2 did worst on tamoxifen in a cohort of 316 breast cancers²⁶. HER2 and MED1 cross-talk regulates tamoxifen resistance²⁷. Signalling through HER2, PI3K and Akt represses FOXO3a and thereby lowers ESR1 expression²⁸. Higher HER2 is associated quantitatively with lower steroid hormone receptor levels in receptor-positive primary breast cancer²⁹, and HER2 overexpression is an independent marker of resistance to endocrine therapy in advanced disease³⁰.

The receptor into HER2. The reverse arm is better evidenced than its neglect suggests. The oestrogen receptor together with PAX2 represses ERBB2 transcription, and the balance between PAX2 and AIB1 at that site determines the response to tamoxifen³¹. Oestrogen and antioestrogen modulate HER2 messenger RNA and p185 protein in opposite directions⁷, and short-term neoadjuvant endocrine therapy upregulates HER2 protein in tumours classified as HER2-negative⁸. The receptor is a transcriptional regulator of the very gene that is usually drawn as acting on it.

Blockade of one raises the other. Oestrogen receptor signalling is upregulated as an adaptive survival mechanism in HER2-positive tumours treated with anti-HER2 therapy³². The chromatin module through which this runs has been mapped to BRD8 and EP400 in hormone receptor-positive, HER2-positive disease³³, and the corresponding chromatin biology is developed in Chromatin remodelers and the BRD8 and EP400 axis. Resistance to trastuzumab and resistance to lapatinib proceed by different routes, with the receptor and HER2 reactivation contributing differently to each³⁴.

Where the two meet on chromatin. The clearest integration point is the pioneer factor. The HER2 and HER3 heterodimer acts together with deacetylated FOXA1 to evade hormone response, by changing where FOXA1 binds chromatin³⁵. Both pathways converge on the protein that decides where the oestrogen receptor is allowed to act, which is the mechanism described from the receptor's side in Genomic ER signaling, enhancers, pioneer factors, and the cistrome.

Because this is a plane rather than an arm, suppressing one axis raises traffic on the other. That predicts dual blockade, it predicts that single-axis treatment selects for the other axis, and it predicts that hormone receptor-positive, HER2-positive disease is a distinct problem rather than an intersection of two Hormone receptor-positive HER2-positive disease and endocrine integration.

8 · Feed-forward loops, adaptive rewiring, and cistromic reprogramming

Adaptation to HER2 blockade happens on three timescales. Running them together is what makes resistance look mysterious.

Hours. Relief of feedback. HER3 transphosphorylation recovers under kinase inhibition through Akt-mediated negative feedback¹⁷. Suppression of PI3K raises oestrogen receptor transcriptional output and receptor dependence³⁶, partly through KMT2D³⁷. Nothing has been selected and no gene has changed.

Days to weeks. Cistromic reprogramming. FOXA1 chromatin binding is redistributed under HER2 and HER3 signalling³⁵, the oestrogen receptor programme is upregulated under anti-HER2 therapy³², and a defined chromatin module mediates the response³³. The genome is unchanged at this stage. The transcriptional output is not.

Months. Clonal selection. Fixed alterations in surviving populations, which is the subject of Temporal heterogeneity and clonal evolution.

One genuine feed-forward loop runs across the first two. The oestrogen receptor with PAX2 represses ERBB2³¹. Where tamoxifen fails to hold that repression, HER2 output rises. Rising HER2 signalling activates AIB1 by phosphorylation, and high AIB1 with high HER2 is the combination associated with worst tamoxifen outcome²⁶. Increased AIB1 activity supports agonist-like receptor output at sites where an antagonist was intended. The loop closes without any mutation.

The measurement consequence is the reason this section exists. None of the above is visible on a repeat HER2 or oestrogen receptor assay at progression, because all of it is a change in activity rather than in abundance. A stable pair of receptor results at progression is entirely compatible with a completely rewired tumour.

9 · Resistance to antibodies, tyrosine kinase inhibitors, and conjugates

The three modalities fail differently, in ways that are the basis for what Part X does next.

Antibodies. Four routes, separable by what each one changes. Loss of the epitope, as with p95HER2, which removes the binding site while leaving the kinase²¹. Loss of the dependent compartment, which leaves a population that was never amplified HER2 loss as an acquired resistance mechanism¹². Activation downstream of the receptor, so that blocking it is irrelevant, through PTEN loss³⁸ or PIK3CA mutation, the latter associated with lower pathologic complete response in early HER2-positive disease³⁹. And restoration of pathway output through HER3¹⁷.

Tyrosine kinase inhibitors. Not epitope-dependent, so a p95-expressing tumour remains sensitive in principle²¹. They fail instead through the HER3 rebound described in HER3, neuregulin, and ligand-driven escape, through the same downstream alterations, and through allele-specific insensitivity where the receptor is mutant rather than amplified^13,14. The mechanisms of resistance to trastuzumab and to lapatinib are not the same, which is why failure of one does not predict failure of the other³⁴.

Conjugates. Three separable points of failure, the antigen, the payload and the transporter, which are set out in Conjugate exposure, antigen loss, payload resistance, and transporter change and Antibody-drug conjugates and targeted delivery. The one specific to this chapter is that antigen expression measured at progression answers only the first question.

The oestrogen receptor as the escape lane. A hormone receptor-positive, HER2-positive tumour has a route that a receptor-negative one does not, because receptor signalling rises adaptively under anti-HER2 pressure³². This is the direct handoff to Part X. It is the mechanistic argument for integrating endocrine therapy into HER2-directed treatment rather than sequencing it afterwards Hormone receptor-positive HER2-positive disease and endocrine integration. It is also why real-world survival in this subgroup has been examined as a function of whether endocrine therapy was given at all⁴⁰.

Convergence on the cell cycle. Several of these routes end at the same node. CDK4/6 inhibition overcame resistance to HER2-directed therapy in preclinical models⁴¹, and has been studied clinically in pretreated HER2-positive luminal disease⁴². The cell cycle biology is in Cell cycle regulation and the resistance taxonomy that absorbs all of this is in Mechanisms of therapeutic resistance.

Telling these routes apart at the bedside would require measurements that mostly do not exist yet. Epitope status, HER3 transphosphorylation and payload sensitivity have no routine assay. Copy number, protein level and mutation do. The gap between what this section blames and what a laboratory can report is the honest summary of where HER-family resistance stands Metastatic HER2-positive disease.

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